1. Field of the Invention
The present invention relates to a color image forming apparatus especially called as a tandem-type color image forming apparatus, which has developing means of a plurality of colors and means which transfers in order images of the plurality of colors formed by the developing means.
2. Related Background Art
In recent years, for achieving a higher image forming speed in an electrophotographic color image forming apparatus, a tandem-type color image forming apparatus which has developing devices and photosensitive drums whose numbers are the same as a number of colors and in which images of different colors are transferred in order onto an image conveying belt or a recording material, is increasing popularity. Such a tandem-type color image forming apparatus is known to include a plurality of disadvantageous factors which causes a misregistration, and various countermeasures are being proposed for each of the disadvantage factors.
One of the disadvantageous factors is an unevenness or a mounting misalignment of a lens of a deflection scanning apparatus, or a mounting misalignment of the deflection scanning apparatus on a main body of the color image forming apparatus. In such case, the scanning line shows an inclination or a curvature. Since the inclination or a curvature depends on a color, their differences for colors cause a misregistration.
As a countermeasure to such misregistration, Japanese Patent Application Laid-open No. 2002-116394 discloses a method, in an assembling step of the deflection scanning apparatus, of measuring a magnitude of the curvature of the scanning line with an optical sensor, then correcting the curvature of the scanning line by mechanically rotating the lens and then fixing the lens with an adhesive.
Also Japanese Patent Application Laid-open No. 2003-241131 discloses a method, in a step of mounting the deflection scanning apparatus on the main body of the color image forming apparatus, of measuring a magnitude of inclination of the scanning line, then regulating the inclination of the scanning line by mechanically inclining the deflection scanning apparatus and mounting the same onto the main body of the color image forming apparatus.
Also Japanese Patent Application Laid-open No. 2004-170755 discloses a method of measuring an inclination and a curvature of the scanning line with an optical sensor, and correcting bit map image data so as to cancel such inclination and curvature, thereby forming a corrected image. This method, being based on an electrical processing of the image data and therefore dispensing with a mechanical regulating member or an adjusting step in the assembling, can achieve a less expensive correction of the misregistration, in comparison with the methods described in Japanese Patent Application Laid-open Nos. 2003-241131 and 2004-170755. Such electrical correction of misregistration is categorized as a misregistration correction for a pixel in the unit of a pixel or a misregistration correction for a pixel less than a pixel (hereinafter referred to as “correction less than a pixel”). The correction in the unit of a pixel is executed, as shown in FIG. 6, by offsetting the pixel in a sub-scanning direction in the unit of a pixel, according to the correction amount for the inclination and curvature. The correction less than a pixel is executed, as shown in FIG. 7, by regulating a gradation level of the bit map image data, in preceding and succeeding pixels in the sub-scanning direction. Such correction less than a pixel allows to remove an unnatural step difference generated at a boundary of offsetting in the correction in the unit of a pixel, thereby smoothing the image.
Further, Japanese Patent Application Laid-open No. 2005-297633 discloses a countermeasure to a density unevenness in a fine image, induced by a correction less than a pixel and constituting a drawback of the afore-mentioned electrical correction for the misregistration. Such density unevenness in a fine image will be explained with reference to FIG. 9. In FIG. 9, an upper part shows an input image, which is a fine line having a certain gradation having a certain gradation level. A correction for misregistration on such input image provides an image as shown in a middle part in FIG. 9, with a toner density as shown in a lower part. Thus, while the input image has a constant gradation level, the output image after the correction of misregistration becomes a fine line having an uneven density. Such a phenomenon is induced from a fact that an electrophotographic image forming apparatus is not adapted for forming an isolated pixel, while maintaining a proportional relationship between a gradation value of image and an actual image density. In a fine image formed by such fine lines, the influence of such a phenomenon appears conspicuously as a density unevenness.
One of the countermeasures to such density unevenness in the fine image is not to execute the correction less than a pixel, on a fine image. More specifically, there a method is known to binarize an image, then to compare the binarized image with a smoothing judgment pattern stored in advance, and not to execute a correction less than a pixel in case the binarized image matches the pattern but to execute a correction less than a pixel in case the binarized image does not match the pattern.
However, such known methods are associated with the following drawbacks. In the electrical correction to misregistration which is one of countermeasures to the misregistration, the density unevenness may be generated in various fine images. Though non-execution of the correction less than a pixel on a fine image is effective in avoiding the density unevenness, it is impractical, in consideration of a memory capacity, to store all the fine images that give rise to the density unevenness as the smoothing judgment patterns.
Also a level of the density unevenness varies depending on a color of the image. The prior countermeasures, not taking image color into consideration, may execute a correction that rather makes the density unevenness more conspicuous.